1. Field of the Invention
The present invention relates generally to signage articles such as retroreflective and non-retroreflective articles. More particularly, the invention relates to pressure-sensitive adhesives which allow excellent adhesion of retroreflective and non-retroreflective base substrates to surfaces such as highly monomericly plasticized polyvinyl chloride coated fabric, as may frequently be used in tarpaulins.
2. Related Art
In the article "Truck Cover Market Report", Data Textile, May 1991, distinction is made between tarpaulins and truck covers. A truck cover is defined as a fitted cover or a cover specifically designed for use on a flat bed, open top, dump truck or trailer, and differs strictly from a tarpaulin per se, since tarpaulins are not usually fitted for any particular use. For the purpose of the present invention, the terms are interchangeable.
As discussed in Patent Cooperation Treaty (PCT) application no. WO 93/10985, published Jun. 10, 1993, tarpaulins usually consist of a fabric coated with a plastic material. The fabric, although not required in all instances, is usually a woven polyester or nylon, which may have a weft insertion. The plastic material is typically chosen to be tough and flexible, and thus plasticized polyvinyl chloride (PVC), polyamides (such as nylon and aramid), and polyprenes (such as chloroprene rubber) are commonly employed. Tarpaulins are used for many cover purposes, e.g., within the building industry, and in particular for covering trucks. They are also used for making larger tents, e.g., for use in temporary military quarters and refugee camps as well as for public events.
As further stated in the above-mentioned PCT application, it is common practice to print logos, company names, slogans, instructions and other decorations on truck tarpaulins, for informative and for advertising purposes. Reflecting decorations are particularly advantageous, making the trucks visible at night. Often these decorations or adherends are applied outdoors to the truck canvases. Thus, suitable PSAs preferably are capable of application over a broad range of temperatures.
One solution to the problem is described in the above-mentioned PCT application, which describes a tarpaulin comprising a fabric coated with a plastic material, preferably PVC, polyamide, or polyprene, provided with a decoration of a reflecting material by anchoring (with an overlaying transparent flexible film) a reflecting sheet to a piece of intermediate tarpaulin cloth, whose coating is of the same type as (or is compatible with) that of the tarpaulin, by high frequency welding or heat application, and then applying the piece of intermediate tarpaulin cloth provided with reflecting sheet and overlaying transparent flexible film, optionally formed in the desired shape, to the tarpaulin by hot air fusion.
PVC coated fabrics differ primarily in the type and amount of plasticizer added to the PVC. One commercially available PVC coated fabric has been analyzed to contain up to 42 weight percent of low molecular weight monomeric plasticizer. These low molecular weight monomeric plasticizers tend to migrate to the surface of the PVC coated fabric, and tend to cause problems in adhering materials to the PVC coated fabric with pressure-sensitive adhesives (PSAs).
Acrylic PSAs generally comprise a primary component of acrylate or methacrylate monomer or a combination of such monomers which, when polymerized, have a low glass transition temperature (T.sub.g)and low modulus (i.e. they are rubbery and soft). These soft, tacky low T.sub.g monomers are typically copolymerized with a secondary component consisting of high T.sub.g monomers, usually polar monomers such as acrylic acid, methacrylic acid, itaconic acid, acrylamide, methacrylamide, and mixtures thereof. As described in U.S. Pat. No. Re 24,906 (Ulrich), when such polar monomers are incorporated with a predominance of low T.sub.g monomers, a sufficiently tacky pressure-sensitive adhesive is formed having high cohesive or internal strength. Further increase in internal or cohesive strength (i.e., shear strength), which is often required to resist the severe environmental and chemical conditions found in transportation applications, can be obtained via crosslinking.
One approach to reducing the tendency of monomeric plasticizers from migrating out of highly monomericly plasticized PVC coated fabric and into the attached PSA is to load the adhesive with plasticizer, thereby reducing the mass transfer gradient for plasticizer migration from the PVC into the adhesive. Such an approach was taken in U.S. Pat. No. 4,946,742 (Landin), which discloses normally tacky and pressure-sensitive adhesives having excellent long-term adhesion to plasticized vinyl surfaces, prepared from a representative blend of dioctyl phthalate plasticizer and a terpolymer of an alkyl acrylate, a nitrogen containing vinyl monomer and a vinyl carboxylic acid. Addition of the plasticizer to the adhesive, however, adds to the cost of the adhesive, and requires an additional process step. Furthermore, if the plasticizer present in the PVC coated fabric is different from the plasticizer present in the adhesive, a driving force still exists for the plasticizer present in the PVC coated fabric to migrate into the adhesive due to the concentration gradient.
Development of a non-plasticized PSA would allow all fabricators to apply cube-corner type and other types of retroreflective sheeting onto monomericly plasticized PVC coated fabrics, thereby reducing or eliminating the need for thermal attachment methods and for plasticized PSAs.
Assignee's published European patent application no. 615 983 A2, published Sep. 1, 1994, describes a PSA having outstanding ability to bond to solid acidic surfaces such as acidic acid-rain resistant automotive paints and PVC, and to remain firmly bonded thereto, comprising (a) about 60 to about 90 parts by weight of at least one monomer selected from the group consisting of monofunctional unsaturated (meth)acrylate esters of non-tertiary alkyl alcohols, and mixtures thereof, the alkyl groups of which comprise from about 4 to about 12 carbon atoms which as homopolymers have glass transition temperatures below about -20.degree. C.; (b) correspondingly, about 40 to about 10 parts by weight of a basic monomer copolymerizable with the monomer of component (a); (c) about 0 to about 3 parts by weight of an acidic monomer copolymerizable with the monomers of components (a) and (b) wherein when the acidic monomer is included, the basic copolymerizable monomer should be present in a molar excess; and (d) about 0.05 to about 1 percent by weight of a crosslinking agent based upon the total weight of (a) plus (b) plus (c). Representative examples of copolymers described therein are copolymers of isooctyl acrylate (low Tg monomer), acrylic acid and a basic copolymerizable monomer which may be selected from strongly basic, moderately basic, and wealdy basic monomers. Although this work is impressive, there was not disclosed or suggested the use of the PSA compositions therein disclosed in binding a variety of materials, such as used in reflective and non-reflective signage articles, to highly monomericly plasticized PVC surfaces.
PCT application WO 94/19711, published Sep. 1, 1994, describes a retroreflective structure in which an array of free-standing retroreflective prisms is formed on a substrate for application of the structure to pre-existing substrates formed of compatible fabrics, such as tarpaulins. The structure employs a non-pressure-sensitive adhesive which requires time to cure, such as a one component moisture-curable polyurethane adhesive, to adhere the free-standing prisms to the substrate, and thus the structure requires preassembly.
It would be advantageous if pressure-sensitive adhesives lacking plasticizer could be used to adhere retroreflective and non-retroreflective sheeting to highly monomericly plasticized PVC surfaces.